Cables for transmission of high frequency signals, such as a coaxial cable and a LAN cable always cause dielectric loss. Printed wiring boards used for various transmission devices for transmission of high frequency signals have the same kind of serious dielectric loss problems.
The dielectric loss is a function of dielectric constant (∈) and dielectric dissipation factor (tan δ). The smaller the two factors, the better. In order to reduce the dielectric loss, several attempts have been made to prepare a high frequency cable in which PTFE having excellent electric properties is used as a material for the insulating coating layer (JP-A-11-31422, JP-A-11-283448 and JP-A-2000-21250).
Meanwhile, the melting point, dielectric constant, dielectric dissipation factor, and in addition, mechanical strength of PTFE undergo some changes due to heat treating (sintering). For example, non-sintered PTFE has a melting point of as high as 340±7° C., a dielectric constant (∈) of as low as 1.8 and a dielectric dissipation factor (tan δ) of as low as 0.5×10−4 (all measured at 12 GHz, also in the followings), while half-sintered (semi-sintered) PTFE has a melting point of as low as 327±5° C., a dielectric constant (∈) of as high as 2.0 and a dielectric dissipation factor (tan δ) of as high as 0.7×10−4. In the case of completely sintered PTFE, the melting point is further lowered to 323±5° C., the dielectric constant (∈) becomes as high as 2.1 and the dielectric dissipation factor (tan δ) becomes as high as 2.0×10−4, while the mechanical strength is improved by sintering.
Accordingly, it is effective to use the non-sintered or semi-sintered PTFE in view of the dielectric loss. On the other hand, it is effective to use the sintered PTFE in view of the end processability and strength.
In such circumstances, the above publications attempted to prepare an insulating coating layer in which sintered PTFE, semi-sintered PTFE and non-sintered PTFE are combined together in order to improve processability while maintaining electric properties such as dielectric constant and dielectric dissipation factor.
In JP-A-11-31442, a method of increasing the sintering degree of the outer surface of the insulating coating layer comprising non-sintered PTFE (differentiation of PTFE sintering degree in the radial direction) is proposed.
In JP-A-11-283448, a method in which the insulating coating layer basically comprises non-sintered or semi-sintered PTFE, and the end portions to be processed (10 cm from the end) is made of completely sintered PTFE (differentiation of sintering degree in the longitudinal direction of the core) is proposed.
Furthermore, in JP-A-2000-21250, a PTFE porous layer is used for the insulating coating layer and the surface of the PTFE porous layer is sintered to adjust the crystallinity to be as high as 75 to 92% (differentiation of sintering degree in the radial direction).
However, with respect to the processability of the end portion of the cable, the end portion of cables using non-sintered or semi-sintered PTFE cannot be cut neatly but becomes fibrous and stringy when the end is peeled or cut by a nipper and the like.
For these reasons, smooth end processing has been considered impossible unless completely sintered PTFE is used as in JP-A-11-283448.
Though sintering degree and crystallinity of PTFE for insulating coating layer of high frequency cable has been variously studied in this way, the molecular weight of PTFE has not been considered. Thus, the present invention focuses on the previously unnoticed molecular weight of PTFE, from the viewpoint that low molecular weight PTFE is less likely to become like fibers at end portions when cut, considering the sintering degree as well, and the present inventors have produced an insulating coating layer material comprising PTFE which has improved processability and end processability in addition to excellent electric properties.
An object of the present invention is to provide a high frequency cable with small dielectric loss, which has an insulating coating layer made of PTFE, and whose end portions can be processed smoothly, and PTFE powder for such PTFE insulating coating layer.